Reducing the emissions of pollutants caused by the operation of internal combustion engines is a continuing goal of industry in general, and the automotive industry in particular. Developments in this area have been spurred on in part by federal and state legislations which set limits for the permissible levels of numerous gasses and other pollutants that result from internal combustion engines such as the gasoline burning engines used in automobiles. For example, California's Air Resources Board (ARB) adopted Low-Emission Vehicle (LEV) regulations in 1990. This set of regulations requires significant reductions in automobile emissions and run from 1994 to 2003. The ARB has since amended these regulations to impose even greater emission reduction requirements. These new regulations, LEV-II, will run from 2004 through 2010. LEV and LEV-II impose very stringent requirements on emissions from automobiles. Other relevant regulations are the U.S. Environmental Protection Agency National Low Emissions Vehicle (NLEV) standards. As a result, the automotive industry is continuously investigating ways to reduce emissions in order to comply with these and other legislative requirements.
Emissions from internal combustion engines include not only the resultant combustion gases, such as carbon monoxide, but also fuel emissions, e.g., the leakage of fuel vapors as the fuel, e.g., gasoline, is transported from the storage vessel to the point of combustion. To reduce such emissions, gaskets and other sealing elements are used to seal joints. Such sealing elements are made from a variety of materials including polymers, fiber composites, graphite, and steel. Typical polymeric gasket materials used in automobiles include silicone rubbers, fluorosilicone rubbers, and HNBR rubber (hydrogenated acrylonitrile-butadiene rubber or hydrogenated nitrile rubber).
However, due to the demand for even lower emission levels, polymeric materials that exhibit even lower permeability to fuel vapors are being used, for example, fluoroelastomers (FKM). Typical examples of fluoroelastomers are copolymers of vinylidene fluoride and hexafluoropropylene and terpolymers of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene. These fluoroelastomers possess not only low fuel permeability, but also excellent heat stability, good resistance to solvents, oils, and other chemicals, low compression set, and good processability. However, fluoroelastomers are relatively expensive materials, and thus there is a need to reduce costs associated with the manufacture of molded articles such as gaskets from fluoroelastomers.
In the manufacture of molded articles from fluoroelastomers, a two step curing or vulcanization process is typically used. First, the articled is molded and undergoes an initial “within the mold” cure induced by the application of heat and pressure. Subsequently, the molded article undergoes a post cure step wherein the article is heated to, for example, 225° C.-250° C. and held at that temperature for a period of time, e.g., from about 12 up to 16 hours, or even up to 24 hours, sometimes even up to 48 hours .
This post-cure procedure greater increases production time and costs. For this reason, the industry has sought curable fluoroelastomer compositions that exhibit low-post cure. One such material is Technoflon FOR HS® sold by Ausimont USA, which is said to provide a 75% reduction in post cure rate. This material is a 66% fluorine fluorocarbon elastomer combined with a bisphenol curative. In this material, hygroscopic end groups are eliminated in the polymer backbone which results in improved compression set because the ionic forces of the end groups, which tend to adversely affect compression set, are mitigated. While this material does exhibit shorter post cure times, there is still a need for materials with even lower post cure times, more particularly there is a need for materials that can be characterized as non-post cure materials.